I managed to get Debian Squeeze (parent OS) and OpenBSD dual booting but it took a custom entry in /etc/grub.d. You also have to run the update utility after you make the entry.
Other successeshttp://forums.freebsd.org/showthread.php?t=9050

This is by recall, I decided that a dual boot system was not appropriate for my main work station and converted it to a straight OpenBSD system. I have not tried NetBSD.

I remember that I installed Debian 6.0 first and did not use any extended partitions. I used Slackware cfdisk to set the partitions and label the 4th partition as A6. I also had success with fdisk in openbsd setpid as A6 on an unlabeled 4th partition.

The OpenBSD installer will recognize the A6 partition and gave the option to use the OpenBSD partition. Of course I did not install the OpenBSD boot loader. After the OpenBSD install completed the default Debian /etc/grub.d entries did not recognize the OpenBSD partition but a custom entry (keeping in mind the different naming conventions between grub1 and grub2) worked.

I also did essentially the same thing with Arch Linux and OpenBSD - The default Arch install uses grub1.

The index post does not give alot details on the linux distro used, how the partitioning was initially done.